Online Ticket Order / Donation Conversion Rate

Hey all!

We're looking to do some benchmarking for online donations that come in with online ticketing orders. Is anyone currently tracking that conversion rate? Essentially we're asking what percentage of online ticket orders also have discretionary donations attached to them (whether just an add-on donation or a round-up)? Would be grateful for any information you're willing to share as we look to figure out what our target should be there. 

Thanks so much!

-Courtney