I have 3 Selfmade osage selfbows, all of the same basic dimensions, all within 1/2lb of draw weight to each other,(50#@28"). I cannot seem to shoot the same wood arrow off each of these bows without making some adjustment to the arrows. I prefer to alter the shaft length and leave point weight the same. I assume very minor tiller/timing difference in each bow to be the cause. Maybe difference in physical properties of the wood itself, growth ring thickness, early to late growth ring ratio, who knows. But I think any formula, would still only be an educated guess, and then would have to fine tune arrow to individual bow from there. JMHO. Brian