If you transition from a Reserve Component to Active Duty, how do you figure out how many retirement points you have?

The simplest answer is to take your point count, subtract the membership points (15 a year) as they are not counted, and divide the number remaining by 30 and that is your number of creditable months.
After that, it is a matter of remaining AC long enough to attain 20 years.
The big step in this process when you transition is calculating your Retirement Year Ending date, or RYE date. Done incorrectly, it can cause you a lot of pain and suffering later on when you are trying to retire.

Only your active duty time counts towards your active federal service. The battle assembly weekends and membership points will get added in, but only after you qualify for a 20 year regular retirement. Thoses points are added up and divided by 30 and credited in whole months towards your retirement pay.

Found this info on a National Guard website, Hope this helps

"1405" Service and Retirement
What is "1405" Service?

1405 Service is actually those Inactive Duty Training (IDT) points creditable toward retired pay that a reservist earned (normally prior to being ordered to continuous active duty service).

Does 1405 Service count when I compute my 20 year active duty service and retirement eligibility date?

No. To qualify for active duty retired pay, you first MUST complete 20 years of active duty – or qualify for, apply for, and be accepted for Temporary Early Retirement Authority (TERA) before you can count 1405 service. Note: TERA is normally offered to individuals in controlled grades who have more that 15 years, but less than 20 years of active duty service

How does 1405 Service affect my active duty retired pay?

To calculate your 1405 service, you will need to know two numbers – your total points for retired pay and your total active duty points. You can get these numbers from the printout you receive annually – Air Guard from your AF Form 526 or Army Guard from your RPAM (NGB Form 23B).

Sample calculation:

Total Points for Retired Pay 7458

minus AD Points - 6773

equals 1405 Service Points 685

You’ll need to convert 1405 points into years
685 divided by 360 = 01 yrs 10 mos 25 dys

In this case active duty retired pay will be calculated on total years of active duty plus 1405 service.

Active Duty 20 yrs 00 mos 11 dys

plus 1405 service 01 yrs 10 mos 25 dys

equals 21 yrs 11 mos 06 dys

CPT (Join to see) You should have a Chronological Retirement Points Statement in your personnel record. Depending on when you transferred, it will either be an AHRC 249-E or a DA 5016 if transferring from Army Reserve or an NGB 23B from the state if you were in the National Guard. If you don't have a copy of this document you can look in your electronic record through the HRC website or contact the state guard headquarters if you transferred from the National
Guard.