I think perhaps you are the best person to answer your question about a sliding scale. What is the reason for the sliding scale? If it is to accommodate and include those who may have a harder time financially, how will you make this determination fairly from client to client (without looking at tax returns). :). It will also depend on rates being charged in your area. Just a few things to think about.
If you can give a little more detail about your plans it may help, otherwise I’m not sure what is fair. I’ve always thought it is too difficult to charge different rates, but you have your reasons and it may work for you. Hopefully someone else who charges on a sliding scale can help you more.
Best and good luck,