Hello Sue (Apodemos) and welcome to the ArtRage forums

Very impressed with your map making skills
Sadly I’m afraid it's not possible at present for the user to adjust the canvas lighting in any version of AR.
I too would like to see AR’s canvas lighting not use any fall-off which mimics a spotlight hitting the canvas but rather mimic Sunlight coming from one direction with no fall-off.
Indeed with the Sunlight model, if a user really then wanted light fall-off on the finished painting it would be quite easy to fake it using a gradient mask with a darkening blend mode across the picture.
I tried experimenting some time back with using a lightening gradient mask, sort of the revers of the above, to counteract the current lighting model but found it difficult to get consistent usable results.
But now you've reminded me I might have to revisit those experiments again when time permits…