MY DA/SA P226 and P229 .40's now have 18# hammer springs. I used to run 19#, 18 years with the P229 and four years with the P226. Dropped a little lower about 7-8 years ago when I started having hand/finger problems due to arthritis. Have never had a failure to ignite with either pistol.
Some here have had problems with RP hammer springs slowly deteriorating in power. I've never experienced that. Replacement cycles should probably be shortened with RP, compared to factory.
You should be able to reliably use 19#, I think GGI uses that in their action kits. Usually, factory hammer springs in most guns are somewhat overpowered, for obvious reasons.
In summary: (a) With MY guns, I have not had a reliability problem with 18 or 19# springs. The 19# would be the safe choice. Other pistols may be different. (b) I'd avoid 17#. IMO, pushing the envelope. My testing with 17# on both guns resulted in slightly funky feel to trigger, although ignition was fine. Don't forget that reducing the DA pull also results in reducing the SA.
Regardless of what you do, thoroughly test to make sure you get reliable ignition with the intended ammo. Especially for SD.
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing. --Nicholas Murray Butler