averaging a sum of cells in a range with ability to accomodate adhoc manual entries

I need a formula that will take a number from column A and divide that total starting number into equal values over a variable series of cells in the same row.

Example column A1 = 100, and the formula is copied and pasted across 10 corresponding columns on row 1, the returned value would be 10. If it is only copied to 5 columns the answer would be 20, 4 columns the answer would be 25 etc. The selected columns could start at column B, or column G, or column ZZ. Where it starts doesn't matter so long as the range of cells copied to is a continuous range. (We want to be able to drag the bottom right corner of a cell to draw the range).

The second criteria would allow a manual insertion of a typed in value which the formula would take into account and adjust the other values still containing the formula. In the example of A1 = 100 and the formula is copied over 10 cells, if D1 was to have 19 entered as a value, the other 9 cells would take that into account and execute a result of A1-19=81...81/9 remaining cells should return equal values of 9 in the 9 remaining cells.

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