To the OP ... there have been several suggestions offered, some better than others and with rules of thumb that will generally work if nothing goes wrong on the dive.
My simple answer would be don't go to 100 feet using an AL80. I'd even be hesitant to suggest going there using an HP100, given what you've said about your relative size and air consumption rate. The reason is that going to 100 feet for a new'ish diver (you indicate 0-24 dives experience) is like driving across a deserted stretch of highway, where you know there won't be any gas stations available for 100 miles. You can look at your gas gauge, but in order to know that you'll be able to make it you need to know not just how many gallons your tank will hold, but how many miles per gallon you get. Then you have to factor in a reserve for worst-case conditions. Otherwise the position of the needle on the gauge only gives you a relative indication of how much gas you have relative to a "full tank", and no way of knowing how fast you'll actually use it up. Scuba diving is like that too ... you need to know not only how many cubic feet your tank holds, but how quickly you will use it up. But in scuba diving, your "miles per gallon" is variable not just with depth, but also with conditions ... both physical and mental. New divers tend to go through their air quickly, and even more so when faced with new or stressful circumstances. New divers are often taken by surprise by how quickly their SPG goes down as they go deeper, even though they were taught that it would happen ... there's a big difference between academic knowledge and practical application.
When I was teaching I would have my students calculate how much gas they would need for a 100-foot dive, based on a supplied dive plan. They would measure their air consumption rates, both under reasonable (typical) diving conditions and under stressful conditions, and then use those measurements to perform the calculations. They often (usually) came to realize why that AL80 tank they were renting wasn't adequate for the dive. And it usually came down not to the expected consumption rate if the dive went according to plan ... but rather to how much gas they would need to make a controlled ascent while sharing air with their buddy if the worst case (OOA) occurred while at maximum depth.
Some things to think about ...
1) When someone goes OOA, they are unlikely to remain calm. A diver's consumption rate goes up with stress ... sometimes it goes WAY up. I'm not talking about a panicked diver here ... just someone who has to react to an unexpected circumstance while at 100 feet, while probably dealing with some level of narcosis.
2) You're going to be working harder than normal making the ascent while sharing air. This means both divers will be ascending with an accelerated air consumption rate.
3) If you haven't practiced making an ascent while sharing air since you did it in your OW class, don't assume it will go without a hitch. A more likely circumstance is that you will not have an easy time controlling your buoyancy. It's likely you'll end up cycling more air through your BCD than you normally would.
4) If one of you goes OOA on this dive, how likely is it that your dive buddy is going to have air to spare? Particularly if you haven't both planned your reserves in advance with the circumstance in mind that you might both need to be breathing off of only one of your tanks.
A good rule of thumb for calculating air consumption is that under stressful conditions your "typical" consumption rate could double ... I've measured some people's rate going much higher than that, but double is pretty typical. For new'ish divers, that could put your stressful consumption rate well over 1 cubic foot per minute (x depth pressure in ATAs) for EACH DIVER. At 100 feet, that would mean that two stressed divers could potentially use up 8-10 cubic feet of gas in the first minute ... and that could be just how long it takes to sort out the situation and begin your ascent. You then have to factor in sufficient gas to get you to the surface ... hopefully with enough in reserve to at least power-inflate the donor's BCD (since they'll be helping their dive buddy remain afloat while they orally inflate theirs).
Safety stops are optional. Far better to make a measured ascent and skip the stop (unless you have plenty of gas left by the time you get there, and are confident enough in your buoyancy control to not have to struggle maintaining safety stop depth). The priority at this point isn't making the stop ... it's making it to the surface before you run out of gas ... keep your priorities straight and mindful of how much air you have left to work with before you decide to stop.
There is no magic formula ... rules of thumb are convenient, but we're all different. The only way to know for sure is to take measurements, and calculate your "miles per gallon" under both normal and stressful conditions. Your controlling factor on when to begin ascending won't be how much gas you will need to surface, but how much you'll need if a worst-case emergency (OOA) occurs at the deepest point in your dive.
For more information, go read this article ...
NWGratefulDiver.com
... Bob (Grateful Diver)