So correct me if I'm wrong, but you want to level the skills of some intermediate weapons before using them to level up the skills of the weapons you are actually using.
Based off the values on the wiki:
R weapons are used at skill level 1 to enhance the other weapons to the following levels
R-grails should be leveled to skill level 4: Cost 1+1+1 R weapons to get 2+2+2 extra skill points.
SR-Grails should be leveled to skill level 5: Cost 1+2+3+4 R weapons to get 5+5+5+5 skill points.
SR non grails to level 4: Cost: 1+2+3 R weapons to get 3.5+3.5+3.5 skill points.
SSR non grails to level 2 if you want to use them to enhance a weapon with skill lvl 19. Cost 2 R weapons to get 3 extra skill points.
For the grails and SR weapons they offer more enhancement at those levels than you spend to get them there. The SSR weapons are worth 35x per level but the max you need is 38 points to enhance a lvl 19 SSR, if there are weapons that go past skill level 20 then further enhanceing the SSR weapon should be worth it.

Of course you should save these enhanced intermediates for use enhance weapons skills that need that many points.

I've only tested this with R-grails so far and it worked as I expected.