Not sure but the guide says to oversend by 10% to have a 99% chance of success

Of course if the def bonus is only 3.5% then sending 110 troops would be succeeding most of the time against that as well.

I would suggest trying 103 troops. If the bonus is not working, you should succeed a solid majority. If the bonus is not working, you should fail more than succeed. You will need to do rather a few attempts to be sure though