Help: Need to express progressions in Excel

[SzczerbiakManiac]

I will try to explain this as best I can. I have a series of numeric progressions (all integers) that I need to be able to calculate in Excel. Here they are:

L	L/2	L*.75	L	(L-4)/3		?	??
1	+0	+0	+1	+0		+0	+0
2	+1	+1	+2	+0		+0	+0
3	+1	+2	+3	+0		+0	+1
4	+2	+3	+4	+0		+0	+2
5	+2	+3	+5	+0		+1	+3
6	+3	+4	+6	+0		+1	+4
7	+3	+5	+7	+1		+2	+4
8	+4	+6	+8	+1		+3	+5
9	+4	+6	+9	+1		+3	+6
10	+5	+7	+10	+2		+4	+7
11	+5	+8	+11	+2		+4	+8
12	+6	+9	+12	+2		+5	+8
13	+6	+9	+13	+3		+6	+9
14	+7	+10	+14	+3		+6	+10
15	+7	+11	+15	+3		+7	+11
16	+8	+12	+16	+4		+8	+12
17	+8	+12	+17	+4		+8	+12
18	+9	+13	+18	+4		+9	+13
19	+9	+14	+19	+5		+9	+14
20	+10	+15	+20	+5		+10	+15

The First column is the level of the progression. The next four columns are progressions I have figured out. The formula for each one is in the header.

It's the last two columns that I can't figure out. How would I express each of those using a formula? I know I can do an Index (Array?), but that seems so clunky.

I suspect the column marked "?" cannot be expressed by a formula because of the +2 at Level 7.

Your input is greatly appreciated.