With longish cycles at 38 to 40 days, I think starting at CD10 is way too early and is absolutely going to drive you nuts because O is probably not happening until CD24 and 2 weeks of OPKs is going to get really expensive.

As others have said OPKs don't worked for everyone because they don't always catch the surge. Some women need to use them twice a day to catch it. Others, like me have PCOS and a higher level of LH anyway and don't necessarily need a large LH surge to O because LH is higher, if that makes sense.

Using OPKs after your temp has risen won't work because a temp rise indicates that O has already happened.

I'd recommend using no-cost methods such as CM and cervix position. The key really is getting to know what is normal for you during your cycle. It might take a little while for your normal cycle to reesatablish itself - in the meantime, get to know your own O signs - believe me, it is very helpful!!!

HTH
TICKLISH