Indoor FTP vs outdoor FTP

Hi, Im into week seven now after committing to a Wahoo Kickr purchase and getting a referral from a friend. It seems that by using the program set up for requirements my FTP is lower on the trainer using erg mode than it is whilst out on the road. My question is should i do a separate FTP test on the road (which would be the 20 min test) and use this to work out my zones whilst i get that much needed break from the pain cave? Cheers for all the enthusiastic work you guys do.