Not sure about this. The main advice is not to take blood samples from the arm where the gel has been applied - if this is where you spread it - as this can give a false reading. Theoretically the levels of estradiol reach a steady state after gel is applied daily or twice daily because some is converted to estrone (sulphate?) in the bloodstream and there is constant interconversion going on between estradiol and estrone (hope this is right - sorry no time to check at the moment - busy gardening!). However we have seen from responses on here that some women do get a "hit" after gel application especially at higher doses - which would also be reflected in serum levels.
If you apply your gel doses 12 hours apart then it would be sensible to time your blood tests to mid way between the two to get as near as possible a true reading. However in any case blood levels of estradiol vary hourly and between morning and night too - so one reading won't tell you much even if you do get it taken bearing in mind the above! Also if you are still having periods due to your own ovulation - this will also affect your readings as it depends where in your cycle levels are taken.
Why are you wanting levels checked? 600 pmol/l would be a good reading for most women.
Hurdity x