Day 6 or 7 , than day 10 or so. 50 a pop is kinda expensive as here they got deals for 3 tests in a package here. One is almost worthless.
I sort of figure you know what the use of it is.

Also heat cycles differ a lot, mine was at her peak at the 18th day. So the 10th day test had no use for me. You sort of figure it would be the standard 13th, shows that is a guessing game. Better you ad another test than to miss the breeding by a day or so. Good luck.