Multiplication and Subtotal

Using your posted workbook...

You don't need to individually reference every cell you want to sum

Try this subtotal formula:
E11: =SUM(E7:E10)

and this total
E15: =SUM(E11:E13)

(the multiplication formulas are OK.)

Does that help?

It depends on what you mean by "right". If you select cell E15 and set the number format to General, you'll see that the true value is 46.9472
Consequently, it's up to you to decide if you round the value(s) or simple display them appropriately.
If you need to round the calculated values...search for Excel help on the ROUND function.

Yes, your formula calculates 8% of the subtotal