The simplest way to do this is to add an Analytics dimension. The idea is to add a dimension with one stored member [Input] and then you can base your calcs against that member so in your case it would be
%Rev: [Input] / ([Input], [ACCT-49999]) * 100
This is the easiest way to do it because it only requires one formula and then any account in the acct dimension will calc against Revenue. The downside is if this is a cube that is already built adding a new dim can be an issue with some existing reports, although in most cases it should be fine. In that case you may want to try to leverage another dim like Scenario for instance, where you might be able to sneak it in.
The other alternative is to put the formulas in the accounts dim, but then you have the issue of having to create each calc individually.
Expense Rev %: [ACCT-69999] / [ACCT-49999] * 100
Wages Rev %: [ACCT-60100] / [ACCT-49999] * 100
etc etc etc
The issue here is you have to create a calc member for each member. Some people find that to be easier for end users than an analytic dim, which may be a little challenging to some users if they do not understand multi-dim real well. So you could go either way. Mocking this up in Excel and using a load rule to add the calc members certainly wouldn't be that difficult.
To be clear though, there is no way to create a single member in the account dim to do this dynamically. There would be no way for the function to know what member you want to use as the numerator.