Hmmm....perhaps you should try not to focus on pressure points first, until you can get the lock first?
When you do get the lock, then try augmenting it with pressure points.
With nikyo, you'll want to dig into LI4. I'm not sure you can always reach SI5 at the same time on a person with bigger hands. Although it could work on someone the same height as yourself.
With yonkyo, it's not really a pressure point. Just anywhere on the radial nerve (somewhere between LU6 and 7) would suffice.
If you have Rick's book, be sure to read it carefully, especially the part where he says not all points used are definable pressure points, i.e. the don't exist on any chart.
Bottomline, not a good idea to get hung up on pressure points as THE technique. It's more a secondary augmentation than anything.