fix: better resolution when calculating stx vested

Author: hstove

stackslib/src/chainstate/stacks/boot/sip-031.clar

@@ -85,7 +85,7 @@
         (diff (- burn-height DEPLOY_BLOCK_HEIGHT))
         ;; Note: this rounds down
         (iterations (/ diff INITIAL_MINT_VESTING_ITERATION_BLOCKS))
-        (vested-multiple (* STX_PER_ITERATION iterations))
+        (vested-multiple (/ (* STX_PER_ITERATION diff) INITIAL_MINT_VESTING_ITERATION_BLOCKS))
 
         ;; If we have completed (or exceeded) the scheduled number of iterations,
         ;; consider the *entire* vesting bucket unlocked.  This avoids leaving a