bb/hr calculation

[tomcain]

Hello

My math is too rusty. I am trying to create a spreadsheet to calculate my BB/hr. Let me give a simple example. Assume I always played .50/1.00 tables. I keep session detail records.
Time won/lost bb/hr
1 hr +10.00 10.00
2 hr -5.00 -2.50

3 5.00 1.67 – is correct

However, averaging the bb/hr column produces 3.75bb/hr, which is wrong. It only gets more complicated and farther off if I play at different size tables. Somehow I need a weighted average that also works correctly across different table sizes.

A more complicated example with different BB sizes

BB Time won/lost bb/hr
1.00 1 hr +10.00 10.00
2.00 2 hr -5.00 -1.25

3 5.00

Average of bb/hr produces 4.38

What would be the correct formula to use in a spreadsheet to get my overall bb/hr?

TIA, Tom