Hello. :)

My bitch has had 2 progesterone tests done, the 1st came in at 1.0ng and today her second came back as 0.8ng I have been told to go back on Monday.
Has anyone had any experience with the levels going down before going up for ovulation?

Yes, not unusual and nothing to worry about.

Do you think Monday will allow enough time for it to raise in level?

I would say you have been given sensible advice and Monday would be the best day to test again.

I also have had progesterones drop a wee bit in early stages too. No need to worry, personally would retest Monday and would be prepaired to test again perhaps wed / thurs dependant on readings. This is my general guide 

0-2  nmol/L           Baseline concentrations too early to estimate ovulation 
3 - 6 nmol /L        Minimum 2 days before ovulation expected however result of 3 - 4 nmol/L may persist for a week or longer before increasing. Earliest estimating breeding 4 - 6 days but could be longer (re test needed)
7 - 12 nmol/L   Minimum 1 day before ovulation. Estimated window for breeding 3 - 5 days but could be longer  ( re test recommended) 
13 - 18 nmol/L   Ovulation impending or just occurred. Estimated breeding window 2 - 4 days  
19 - 31 nmol/L   Ovulation recently occurred. Estimated breeding window 1 - 3 days
32 - 64 nmol/L   Ova have matured, optimal potential for fertility.  Estimated breeding window  0 - 2 days
65 - 90 nmol/L   Ova have matured but aging, decreased potential for fertility.  Breed  at once
Above 90 nmol/L    Too late

Tested today she was 3.5 If I remember correctly. They said she should be ready to go to stud Thursday-Saturday but they want me to test again Wednesday because she dipped last week.

that sounds about right as 3.5ng is 11.13nmol = so estimate 3 -5 days for mating but Could be longer. I would retest as requested
K