Jonah, you can never predict what final pressure will be received at some LDS. Thus, "exact mixture" becomes moot. Wait a while. Read the "Hacker's Companion", wait some more. Continue to buy fills locally.Meanwhile, assess how much gas diving you are actually doing. Calculate the tangibles like a precision oxygen fill whip, and the other stuff mentioned by Pug. Also, consider the intangibles and hassle factors on both sides.
If you and your friends do a lot of diving, then renting O2 tanks may make sense. However, after a while you may get tired of partial pressure filling. For example, when refilling partly empty tanks you will have to consider whether dumping the old gas or calculating the percentages for a top off. PP filling is a SLOW process. Not hard to do but all the little stuff adds up in the intangibles side--as does the hassle of scheduling and running to a dive shop, not to mention the tangible problem of impulse shopping while there.
Look inward. Just what is your committment to diving? How old are you? What is your financial situation? Can you see yourself still diving 20 years down the road? If so, then it might be possible to justify a capital investment in an air compressor. That doesn't mean that you should purchase a compressor, not at all. However, with a compressor it is possible to leverage the whole situation by building a NITROX stik and running premixed NITROX through the compressor. At this point, inspections of various kinds can be done by you. The obligation to display an "oxygen clean" sticker and various other labels becomes optional. In effect, you and your friends can become a cooperative, sharing costs and various duties.