how much equity to invest during a decline, if you want to be fully invested after a 50% sell off

decline	equity to invest	start equity	equity after decline	% of original equity invested
2.5	2	100.0	98.0	2%
7.5	10	97.9	88.1	10%
12.5	17	87.6	72.7	15%
20	28	71.4	51.4	20%
35	42	47.2	27.4	20%
50	100	21.1	0	21%

Since 1928 there have been 294 pullbacks of at least 5%.  

94  (>10%)

43  (>15%) 

25 (>20%) bear market

So in a statistical sense once you hit the 5% threshold your chances of a 10%, 15%, and 20%+ drop are as follows:

10%: 32.0%

15%:  14.6%

20%+: 8.5%