I would try a search at EWI and TWI

Some of their stuff is free and some will cost membership dues, which is a bargain if you do wierd stuff.

If your gas supplier is a major player they might have access to that kind of data. Tell them you are shopping around for new gas suppliers and that technical support is part of your decision making process. This will often get them quickly on track if they have the resources.

The Aluminum Associaton Theory and Practice manual encourages Ar/He mixes for situations like yours and says Helium percentages from 50-75% are preferred.

For TIG (constant current), helium raises the arc voltage, hence the arc power (watts), and provides a huge increase in thermal conductivity, both of which contribute to additional heat input and arc efficiency.

For MIG, I assume you would also have a "linear" contribution to thermal conductivity, but I don't know what if any affect helium has on the arc power since voltage is constant.  Maybe helium in MIG also requires a higher arc voltage for a given wire speed (amperage) and arc length.

Any way, unless you are mixing your own, wouldn't you use a standard 75/25 mix?

Ed Craig suggests 60% Argon 40% Helium His website is WeldReality.com, You might check it out.