If you are shooting a lever advance back or the like, just advance like it was 120 film. If it's a wind-on like in a folder, you'll have to guess. Unload in the dark onto your developer reels.
Or find a Yashica 635 adapter for 135 film and make it work in your camera with standard 135 cassettes. You'll still have to unload in the dark and wind the film back into the cassette but it's easier to do color processing down at the Photomat that way.
Or get out your 8x10 Deardorff with the splitter boards and shoot 4x10... yippee!
tim in san jose