Jon, you must have made a mistake during calculation.
See table, whatever number you pick, you will end up with letter D

Pick a number	subtract 5	multiply by 3	square it	add digits	subtract 4 if > 5, otherwise add 5	multiply by 10	subtract 6
1	-4	-12	144	9	5	10	4	D
2	-3	-9	81	9	5	10	4	D
3	-2	-6	36	9	5	10	4	D
4	-1	-3	9	9	5	10	4	D
5	0	0	0	0	5	10	4	D
6	1	3	9	9	5	10	4	D
7	2	6	36	9	5	10	4	D
8	3	9	81	9	5	10	4	D
9	4	12	144	9	5	10	4	D

MudGuard