I am new to MDX and despite my research I'm struggling with an MDX formula. I'd like to calculate the % of lower level accounts to the sales account ("%REV"). This will be used as a member formula and will be used in the following example:

I can probably help with this, but I don't understand your example.

You show Jan Act for 49999 is 100 and %Rev is 100%, then there are other members that I assume are siblings. If you sum the siblings you get 170%. In Feb the Act number goes to 90 but still stays 100% and for 60110 the number stays 60 but the percent goes to 67%. I have no idea how the math is working here. What are your numerators and denominators?

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.

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.