Can you please advise me on this as I’m not following any pattern, I’m working from my tension swatch.
You could try knitting 2 tension squares using a full number difference between the two. ie say tension for fairisle is 7 try the stocking stitch at tension 6. Unless you the full jumper as described in the first comment it may be difficult to achieve the same feel to the garment but should be able to achieve a resonable tension comparison. hope it goes well
If you want to avoid having the top thicker than the body you could knit the main part in a 1×1 or similar alternating Fair Isle using your main yarn in both feeders. If you decide to do this knit a new tension swatch as the stitches and rows will probably be different than stocking stitch.
Hope your sweater is a success.