Wargame the scenarios. You wouldn't want to give one dose on purpose, but reserving a whole bunch of doses to prevent some people from getting only 1 in a worst case scenario is silly.
That 20-70% efficacy number includes the entire period between dose 1 and 2. It looks way better 10 days after dose 1, btw-- MLE around 80%.
e.g. single dose -- 50% efficacy (to be pessimistic); double dose -- 95% efficacy. 10M doses "in flight" monthly. 100 deaths/million/month. Ignore all effects on transmission (to be more pessimistic).
Scenario 1 (no logistics disruption, half held back): 5M dosed with 1 dose at day 0, 5M dosed with 2 doses at 1 month and 5M dosed with 1 dose at 1 month. 250 saved in first month, 725 saved in second month.
Scenario 2 (no logistics disruption, 25% held back): 7.5M dosed with 1 dose at day 0, 7.5M dosed with 2 doses at 1 month and 2.5M dosed with 1 dose at 1 month. 375 saved in first month, 837 saved in second month.
Scenario 3: (100% production disruption, half held back): 5M dosed with 1 dose at day 0; 5M dosed with 2 doses at 1 month. 250 saved in first month, 475 saved in second month.
Scenario 4: (100% production disruption, 25% held back): 7.5M dosed with 1 dose at day 0; 2.5M dosed with 2 doses at 1 month; (5M late for dose 2); 375 saved in first month, 612 saved in second month.
With a 25% reserve, you only get into the unexpected-only-1-dose regimen with a supply disruption of more than 50%... and even then it's still probably better than a 50% reserve.