"An interesting variant would be to move a spare from the midpoint to the 6pt. Then I would be tempted to play 24/20 6/4 instead"
I would have played 24/20 6/4 too, unstacking the 6pt instead of the mid. I think it is probably still the wrong idea, the safer play, two up from the rear wins again.
  White is XG Roller+
score: 0 pip: 137  Unlimited Game  pip: 156 score: 0
Blue is ian dunstan  
XGID=BEbBcDbbdbB:0:0:1:42:0:0:0:0:10 
Blue to play 42 
1.  Rollout^{1}  24/22 24/20  eq: 0.682 
 Player: Opponent:  34.25% (G:6.05% B:0.21%) 65.75% (G:20.67% B:0.45%)  Conf.: ± 0.011 (0.694...0.671)  [100.0%] Duration: 2 minutes 20 seconds 

2.  Rollout^{1}  24/20 6/4  eq: 0.721 (0.038) 
 Player: Opponent:  33.83% (G:7.38% B:0.28%) 66.17% (G:22.84% B:0.81%)  Conf.: ± 0.014 (0.735...0.707)  [0.0%] Duration: 2 minutes 17 seconds 

3.  4ply  8/4 6/4  eq: 0.747 (0.064) 
 Player: Opponent:  31.65% (G:7.13% B:0.20%) 68.35% (G:18.22% B:0.89%)  

4.  4ply  24/22 6/2  eq: 0.780 (0.098) 
 Player: Opponent:  33.19% (G:7.38% B:0.22%) 66.81% (G:23.90% B:0.81%)  

5.  4ply  24/22 13/9  eq: 0.797 (0.115) 
 Player: Opponent:  32.42% (G:7.12% B:0.24%) 67.58% (G:22.48% B:1.03%)  

6.  4ply  13/9 6/4  eq: 0.798 (0.116) 
 Player: Opponent:  30.93% (G:7.20% B:0.24%) 69.07% (G:18.90% B:1.22%)  


^{1} Truncated at 10 moves using 4ply. 216 Games rolled with Variance Reduction. Dice Seed: 94696176 First 3 moves: 3ply, cube decisions: XG Roller Remaining moves: 3ply Red, cube decisions: 3ply

eXtreme Gammon Version: 2.10