Help needed with workflow

Hi all, just a quick asking al the ones of you out there what their workflow is regarding having a main design an a logo on the same side of a clothing ? Since the mods told me they’re not working on the ability to add multiple layers on one side, how do u guys work the correct placement/aspect ratio/ size when designing your T-shirt’s and jumpers ? What canvas do you start from ? Any help would be much appreciated as currently I’m struggling with having to redo all my artwork many times to trial and error until I get the right placement for my logo by the left hand breast and my main logo being at the right size and position on the garment.

Thank you

Alternatively you could also think of having the Logo right under your design or having your logo on another print area.