Since I do my own fills on a partial-pressure TMx station, and have two sets of doubles, I will frequently blend a "best mix" for a given dive. Exactly what this mix is will depend on a few things:
1.) Depth of planned dive
2.) Likelihood of planned dive getting blown out
3.) Depth of NEXT planned dive
4.) What gas I have in my other set of doubles
5.) Likelihood I can get back to the shop to refill for subsequent dives.
So, for example...
If I'm mixing gas on Friday night for a Saturday morning dive to a wreck that's 75fsw to the sand I'll blend up some 43% for that dive if the weather looks clear and we have a high likelihood of the dive happening. However, if the wind is blowing and weather looks iffy I'll take a look at the depth of NEXT WEEK'S dive before filling. If next week's dive is to 130fsw, I will then look at what other gas I have on hand. If my other set of doubles is filled with 28%, I'll go ahead and mix 43% "at risk" for tomorrow's planned dive to 75fsw, since I know I have the right mix for next week's dive on hand if tomorrow's gets blown out. If, on the other hand, my other set of doubles is filled with 36% then I'll plan to dive that for the 75fsw dive and then fill whatever I need next week. If the other set has something like 32% in it, I might blend 43% for tomorrow figuring I can easily bleed off some of the 32% and refill should I get blown out tomorrow and then need 28% for my next dive. Now suppose the "subsequent" dive to 130fsw is SUNDAY, and I won't be able to get back to the shop to refill and/or bleed off the 32% and refill to 28%? Well, then I'd probably dive the 32% on Saturday and blend some 28% for Sunday.
Pretty straightforward, huh?
I also do a lot of work with students in the quarry, with a max depth of 60ft. For this I will fill up both sets of doubles with 50% for an OW weekend, figuring the likelihood of doing multiple CESA runs, ascending to catch runaway students, etc I'd like to keep nitrogen loading as low as possible.