The main purpose of tillering, whether it be with osage or mulberry or any other bow wood is to get a good even bend. If you can do that on your first bow, no matter what weight you hit, you have succeeded. I think too many beginners try too hard to make their first bow the best ever instead of learning the process. You have to learn to crawl before you learn to walk. Same here! Learn to make bows that that have limbs that bend evenly and together first, no matter what the final weight comes out to.
Once you get the bending worked out in your brain you can work on hitting the weight you are aiming for. Proper tiller is the main concern of a wood bowyer. The weight thing will work itself out as you get familiar with the process.