Stage 1
1 Unit on Zero
+1 Unit on every doublestreet hit, until 36 units are bet.
(Another option is to track the doublestreets for 36 spins)
Stage 2
When total bet is 36 units, Remove the 2 doublestreets hit least. Continue progression +1 on doublestreet hit (on those 4 already placed) When 36 units are placed, +1 on Zero, and continue progression until 72 units are bet. (all bets) Then restart
OR if you have been tracking the doublestreets for 36 spins:
1 Unit on Zero
1 Unit on the four hottest doublestreets. (+1 for each hit, until 36 units are bet, then reset)