Creating An Instance Lighter Than the Lightest Master

Are there inherent issues with trying to create an instance that has a lighter weight than the lightest master?
For example, let’s say I have 2 masters: a Light and a Bold. I would like to create an Ultralight instance based on my two masters. For the most part, it looks like it works, but it seems to be interpolating some things strangely. For example, if I make the crossbar of the Bold master H thicker, it actually makes the crossbar in the Ultralight thinner. Shouldn’t it be the opposite? Its almost as if the Ultralight is referencing the Bold master more than its referencing the Light master. Do I have some sort of setting mixed up in the Font Info panel? Is it a mistake to try to create an instance that isn’t a weight in between my two masters?
Any advice would be greatly appreciated.


This is what happens if you extrapolate. It is like on a seesaw if you push down on one end it goes up on the other.

So would you advise to instead draw an Ultralight master and have the Light be an interpolated instance?

Thanks again for your quick replies.

That depends on your design and how thin the ultralight is. If you have the same contrast in the light and the ultralight you should be fine.
Generate the ultralight instance as a new font (File menu > generate instance) and add it as a master in the original file. Then you can delete the light master.