I ordered P75 etc all having 15-20 mcg strength. Is that a problem of too strong? I am 40 ish year.
Can i spray half a spray to weaken it? Advice?
I am going to try P75 + P96 + ammo. Comments on that?
I’d say it is likely to be too strong and P75 is a difficult one to offset by using other mones. A few people can use P75 at high dose though but IME, this is one where less is most definitely more.
Large quantities of IO is about the best moderator I’ve found but at that dose the number of sprays required will get expensive fast and will upset the balance/proportion of any other mones you may use and possibly wipe others out completely.
I’d say your best bet is to dilute the P75 down with cologne - there are umpteen threads here where the process/working is explained in depth, so maybe search them out.
Age is a slight factor with P75 - it maybe explains why I can use 5-7.5mcg effectively compared to younger guys but its not as clear-cut as with other mones. I’d say that diluting down to approx 2.5mcg is probably the best starting point.
Spraying half a spray is possible but its not that accurate and needs careful finger control on the plunger. If you have an empty togo or bottle, try filling it with water and have a few practise sprays to get the feel without wasting mones. 
P75 complements a lot of products well, esp those with a social leaning, wher it is a useful alternative to None around women who may not respond so well to it. So maybe consider P96/P75 or Ammo/P75 before you mix all three.
With P75 My sweetspot is only 3MCG, but I have seen some others wear very high doses of this one, so it can vary widely on the dose and you will have to test to see what is best. What I would do is maybe test around some people you know and are comfortable with first to see how they react and if you get anything negative, I would wash off and start over.
I have not tested P75/P96 setup, I have tested P75 with P74 and the P74 did buffer the P75 a little when I was on the borderline of OD, but barely TBH.
I almost always have Enone in my daily mix and sometimes swap it out for P75, just to change up my signature so people don’t get to use to a certain sig.
I find with this one that it is good sometime a good replacement for Enone.
What is your goal, and maybe I can give better advice?