CP, Critical Power, 20mins.
FTP, Functional Threshold Power, 60mins.
FTP typically calculated as CP x 0.95. If pure diesel endurance machine I'd use that calc, for most average folks I would say use x 0.94 and some use x 0.93 if you now your endurance is lacking somewhat.
20mins typically used as basis to establish estimated FTP 60mins as getting somewhere and conditions to do a full 60min solo is a ball ache. Unless it's a 25TT where you will have the perfect chance to get a full near to 60mins power down. But 20min as above can be fully controlled and done on turbo on a regular basis to monitor progress etc.
I personally don't think CP8's and CP12's (8min and 12min) should be used as a testing protocol. They should be used as @ CP sessions or FTP+10% sessions. 20 mins is short of the majorities 10TT time so has some relevance to most people regards being an actual amount of time you may spend at that threshold. 8 and 12 minute is just too short then calculate CP and FTP from in my opinion so really cannot see the benefit in it for testing.
316watts @ 75KG as your CP should roughly work out to around 4w/kg (300watts) for FTP. Based on your numbers and for forward training a good goal that is tough but achievable would be to drop your weight to 75kg (not saying you need to lose weight!! purely because you mentioned it!) and achieve your 300watts for a 20min test. Dropping to 75KG alone boosts your W/KG so much, if you can maintain and add 10-20watts as well as drop the weight over the next 8 weeks, you are doing something very right!
Enjoy it and good luck