Perhaps give her an instruction that shows you her enthusiasm for continuing, but allows her a little control, like “hold your bottom up!” Unless she’s completely restrained, she should be able to shift her hips in such a way as to present you with her bottom. At first, she’ll probably do so eagerly and you’ll know she wants more. As things get more intense, and she’s wiggling away or shifting her hips to deflect the blows, you can stop and say “hold up your bottom!” Or “bottom out!” Or whatever reminds her to get back in the right position. If she’s nearing her limits, she’ll get there but she may take a little longer to assume the position than when you first started. She may wiggle more or duck and stretch before getting back into position. Watch for those signs and know that likely means she’s at the top of the range. Either stay at that level for a short time and then slowly back down, or offer her another break and see if she returns to her enthusiastic positioning afterward. If she’s struggling or reticent to offer you her bottom, it’s time to back off or start a cool down. Doing it this way she is giving you a fair bit of information and has a little bit of control from her position without feeling like she is directing you or telling you how to paddle her. Similarly, a lot of people use traffic light terms in place or or in addition to a safe word. If she’s fine and wants more she can say Green. If she’s maxing out and needs a break soon, Orange. If that last one was too much, Red! You can discuss before hand if red means a hard stop, or if red means back down or take a break until she says green again.
When I flog or paddle my husband, I’ll ask a simple “more?” every once in a while or take a little break to rub his ass with my hand and whisper in his ear, “you like that?” When I get an eager “yes!” Or a muffled “m-hmm” through a bitten lip I know it’s ok to keep going. When the chipper yes please turns into a more questionable “um...” or he has to think about it a moment before answering, I know I’m nearing his limits. I don’t want to hit his absolute limit to where he’s no longer having fun, I like to find a good 70-80% as my max then slowly ease back down so he’s eager for more another time!
Remember to talk about it after! Do some aftercare, whatever you both find comforting. Ask if she had enough, if she wants more next time. Ask her what she liked most and least and take note. Tell her how you felt, what you liked, etc. Make mental notes on what she wants more of, or if she felt like she could take more. If you have a hard time continuing when she’s crying, you need to talk to her about that. Explain that it goes against your nature to continue if she’s crying, so her feedback is really important.