I'm a little late replying to the original poster, but as a reservist (ANG and AFR) who is now collecting an active duty ("Regular") retirement and wasn't an AGR, I can shed some light on the process.
It takes 7305 total active duty points to qualify for an active duty retirement. As such, at some point many years ago, I made it a point to take every day of active duty whenever possible, including MPA, RPA, ADSW, and some AGR tours. All of that counted towards my 20 years (including 4 years of active duty in the Army) as well as my time in UNT. I went over 20 years of active duty while on a long-term MPA tour while assigned as an AFRC Traditional Reservist. The hard part was the sanctuary waivers as you're required to have one for each tour when you exceed 16 years (I know sanctuary doesn't kick-in until 18, but AFRC wants them when over 16 years). Also problematic was 1095, which required a waiver and makes it difficult to do tour over 3 years. There are ways to get around 1095, but it's difficult.
Once I exceeded the 20 years and ended my MPA tour, I went to being a traditional reservist again, and had to stay a few months longer to meet the timeline to transfer my GI Bill to my kids (I missed that in 2013 b/c I was in a "No pay, no points" situation for a bogus medical reason). In any event, I applied for an active duty retirement through vPCGR (or whatever it's called now) and retired late in 2017. In addition to having about 20 years and 6 months of actual "active duty" points, I was credited with another 25 months of service for my guard and reserve time (and any correspondence courses). It's called "1405 time" and I believe it references the section of law or the CFR that allows those non-AD points to be computed into your active duty retirement. So, in the end of it, I did about 30 years of total service, with 20 years 10 months of actual active duty and had another 800ish points added to that total to come up with my multiplier based on a total as if I had served for 23 years and 6 months on active duty (~58.75% or something close to that of my high 3). I did lose some of my guard and reserve points for the years in which i did a lot of active duty (i.e. I was an AGR for 3 years, so couldn't use those 45 membership points earned during those three years in which i was an AGR for the full 365 days). Also, all active duty retirements are based on a full month of service, meaning, I lost about another 22 points, b/c my total was something like 23 years, 6 months and 22 days of service. Needed to do another 8 points (active or reserve points) to get credit for another month and a higher multiplier (not much -- 0.21%).
Also, my "high 3" was based on my last 3 years in the reserves, and NOT the last 3 years I was on active duty. If that had been the case my high 3 would've been based on the pay for 2015, 2016 and 2017 as that's when i was on active duty. I remained a traditional reservist for almost 7 months in 2017 and didn't do much if any active duty then. I was told my high 3 would be based on when I actually performed AD, but instead, it was computed using the 9 months I was in the reserves in 2017 (of which 7 were NOT on active duty) and then all of 2016, 2015 and 5 months from 2014. The point to this is, even if you can't get the AD retirement right away, don't think they're going to go back and use your AD from 3 or 4 years ago to compute your retirement.
One last thing: I'm a GS employee with a non-DoD agency and I'd bought back 13 years of active service when i got hired in 2008. Once I retired my agency contacted me to tell me they were going to refund my deposit, b/c you cannot use the time twice if you're collecting an active duty retirement. Just keep that in mind if you're an Technician or ART. Without a doubt the AD retirement is much better than buying back the time (in my case), particularly with TRICARE. I don't plan on staying too long at my federal job, and having health insurance is a HUGE benefit in allowing me to do what I want going forward (and you only get it with the Fed Gov't as a civilian if you take an "immediate annuity" which can significantly reduce your pension depending on your age).
Hope that helps -- it was a confusing process and there's a lot of misinformation out there.