I would just use an extra increment for the Ease and use it in a formula where I need it.
Here is an example of how I use Ease: (((highbust_circ/2)+#Ease)/4)+2
So you have your normal measurement. This needs to remain true to the actual measurement taken from the person for other patterns using different fabrics - you may want to use denim fabric with very little stretch in the next garment and need to add 3cm ease instead of the negative ease for high stretch gym clothes. Then you have your Ease element which you add or subtract from the measurement in the formula and then you have what you want to do with it.
So once you have entered your measurements, they don’t change. You’re only creating more elements that can go wrong. You may accidentally take the actual measurement instead of the one that you’ve taken the ease off and when you do notice the error - normally when you have cut out the fabric and have sewn up your test garment - it will take you hours to find where you went wrong and correct it, not to mention the waste of the test garments fabrics, sewing time, thread, etc. etc.
As I mentioned before, it is always better to create your measurements in SeamlyME and only use the increments for garment specific things like Ease. This way, you can use a persons unique measurement file to draft any garment for them (Blouses, dresses, skirts, trousers, underwear, etc.), simply by changing the measurements file in Seamly2D and adding a few things to your basic patterns.
If you are having problems with SeamlyME, you can check out my blog which has my Youtube videos embedded in them. You can right-click on the page and translate it to any language that you prefer. (You can do that here, too.)