I am creating a Wages, Hour and Payroll summary for my fiancé because he's trying to keep track how the new company is handling wages/time schedules, etc.

However, I'm struggling with a few calculations that I just cannot find solutions for online.

Basically my sheet is set to:
F G
10 Hours Overtime
11 Monday 8.5
12 Tuesday 8.5
13 Wednesday 9.5
14 Thursday 12.5
15 Friday 9.5
16 Saturday
17 Sunday
18 Total Wkly Hours:____ ____

The formula that I have in F-18 is as follows: =((F12-0.5)+(F13-0.5)+(F14-0.5)+(F15-0.5)+(F16-0.5)+(F17-0.5)+(F18-0.5))
It totals the hours he works then minus' his non-paid 30 min break
The problem that I am running into is:
1 - Saturday & Sunday - I want the formula to bypass these cells (F16 & F17) if they are blank

Now for the Overtime column (G11):
Basically I have this formulated to calculate his hours worked minus his non-paid break, then determine anything over the figure in F11 (formula: =(F12-8.5)
but if the F column has no figures, it is still calculated and gives the G column a negative value :/

G18 just sums the figures in the G column and gives him a weekly total of overtime that can be applied to personal and/or holidays.

The problem is, if cell F16 and/or F17 are blank they are still computing the formula, I want it to ignore what is NULL/Blank

I know my formulas aren't very complex and could probably be done much better, but I am not an expert nor every day user of Excel. I can normally make calculations do what I want them to do just by googling information but in this case.. I cant, so if someone could please help me I would greatly appreciate it.

Thank you
Sho