I use +1.5 readers for the computer and +2.5 readers for reading documents (books, forms, paperwork).
So, when selecting a mask, I purchased a mask from Aqua Optics where the entire lens is ground to a +2.5 magnification. Actually, when I ordered it from my local scuba shop, I didn't know that's what I was getting. I will tell you that the first time you put on a mask where the entire lens is a magnifier, it will likely freak you out. You'll think that it will not work for you at all. But, under water, I can assure you it will be fine. And, over time, the magnification in air will bother you less (don't know if you ever get completely past it).
I also, purchased a standard mask from Sherwood (which looks identical to my Aqua Optics mask) and installed +2.5 reader lenses (Aqua Optics) at the bottom of the standard mask lens to use as a backup mask. Those work fine for reading gauges and you don't end up having a lot of money invested into a mask with a custom ground lens.
If you normally use a +2.0 or +2.5 for computer or book work, a +1.5 will work fine for reading gauges. You don't really need as strong a magnification as you'd normally use out of the water because you get some natural magnification from the water itself.
The best recommendations I can give are these:
1) If you decide to go with the small lenses that you glue to the bottom of the standard mask lens, go with the maximum magnification you normally use out of the water (but no more cause you don't need them). You may actually want to try these first to determine the minimum magnification you need to be able to read your gauges. Then, you can purchase a mask with a custom ground lens based on testing different glue-in lenses.
2) If you decide to go with a custom ground lens (i.e. the entire lens is a magnifier) like I did, no NOT go beyond what you'd normally use out of the water AND try to go with a lower magnification if you can get by with it. I say that because I can promise you it will bother you when you're wearing your mask on the surface (i.e. surface swims, water entries from a boat, getting back onto the boat after a dive).
