Unfortunately I do not have alt.app or am. app t-shirts, but I can give you my measurements.
I have a 44" inch chest - actual measurement and I wear a Large in Hanes PerfectT T-shirts. In Sunspel terms, I wear an XL in the T-shirts and XXL in the Riviera Polo (I have 2) and they are starting to get a little big on me...I've been in the process of losing weight, so I will be ordering Large. I will be losing an additional 15 pounds in the next 2 months. So these will be my "target weight" t-shirts. FYI, the Sunspel T-shirts are a little slim.
If you're not sure, order both sizes and either return or sell the other shirt.
Hope this helps!