Calculate cumulative depreciation over time with a dynamic column range

The attached model crudely calculates the cumulative straight line depreciation of new assets over a 15 year period.

The input cell B3 contains the life of assets which is referenced by the depreciation formulas in row 3.

Cell C3 = SUM($C2:C2) /$B3 = 1st year
Cell D3 = SUM($C2:D2) /$B3 = 2nd year
Cell E3 = SUM($C2:E2) /$B3 = 3rd year
Cell F3 = SUM($C2:F2) /$B3 = 4th year
Cell G3 = SUM($C2:G2) /$B3 = 5th year

The 5 year cell range moves for each subsequent years as follows:

Cell H3 = SUM($D2:H2) /$B3 = 5th year
Cell I3 = SUM($E2:I2) /$B3 = 6th year
Cell J3 = SUM($F2:J2) /$B3 = 7th year and so on

I wish to be able to replace the formulas in row 3 to allow the 5 year range of cells to dynamically change according to the input cell B3 and to ensure the range doesn't start before year 1.

I've attempted this with various permutations of SUM, INDEX, OFFSET, COUNT and COLUMN functions without success. Please help.

depreciation.xlsx

Thanks for your quick response Moo, that's spot on. Any chance you could give me a brief explanation of how the formula logic works?

Thanks Moo. That makes sense now, running through the logic.