So to figure out how to do it you need to figure out these few things.

1) Pressure ratio, This is simple 1bar+boost/1bar = X or 14.7+boost/14.7 = X

EXAMPLE: 15lbs 14.7+15lbs = 29.7/14.7 = 2.02
So 15lbs equals 2.0 roughly on your comp map.

2)VE(volumetric efficiency) This can vary on opinion(some say more some say less) but I run 90% for a few reasons.

3)Displacement (again opinions very) but plug in the number 2.6 and it all works good. Plus you will be using a rotary equivalency factor later in your math.

Once you know those things you can plot a point on the compressor map and see where you land simply by adding RPM into the equation.

Displacement x RPM x VE x PR / 5660 = CFM

So 2.6L x 8500RPM x 90%VE x 2.0PR /5660 = 702.82 CFM or 49.25 lb/min

Plot that on a 35R comp. map and you see that your in a mid 70% efficiency at 8500 rpm. Not too bad.

If you wanna get a guess as to WHP, way back in the day I was told 1lb/min = 10hp It seems somewhat true, but basically you are slightly more than that at torque peak and slightly lower than that at HP peak. So we take the middle ground. So all you need to do is move the decimal over one place 492.5whp (that would be piston motor) We don't have any of those.

To figure it out for a Rotary you would need to do it a little different. 1lb/min equals a little over 7.5hp.
I use 1lb/min= 7.69hp and it seems about right. (If you want to be a little conservative just use 7.5)

This is a ESTIMATED number that I have seen to be pretty constant over the years of comparing cars I have worked on and owned. Others who have put their math up on here only confirmed this for me to some degree.

So 2.6 x 8500 x 90 x 2.0/5660 =702.82 or 49.25 lb/min x 7.69hp = 378.73whp@15lbs