I am about 6'5", slim. I wear a 43 jacket. Sleeve length 37". I bought LT from you and found that it was skin tight and a bit short on the length and the sleeves, especially after washing it. I really like the style, quality, and colors (though limited). I am willing to buy another round of shirts but think I need to size up to XLT in extra long.
Here is what I think needs fixing. I mentioned this before: I have no idea what your sleeve lengths measurements mean. EVERYONE ELSE in the business will say sleeve lengths are 36" or 37", etc. Your fit guide doesn't help much. Also, as to body length, I understand current fashion is short length and tailored fit. But as a tall guy, you know that body length is different for short-waisted and long-waisted guys. I'm short-waisted and the length was too short for me (I tuck them in and put on a sweater; too short to wear alone).
So, I admire your start-up. It fits a need. You do need to fix your measurement guide and product descriptions. I kept the ones I ordered and use them as undershirts with sweaters. I would like to order a couple more with a correct fit, but I'm guessing as to the size.
In my opinion what is needed in this narrow market is quarter zip sweatshirts that fit us tall slimmer guys. I buy stuff at Westport and they usually are way too long and fit like a tent. There are plenty of hoodies on the market, such as American Tall (their products are very tight and very long). Perhaps I am a small part of the market and everyone is happy with what you offer, but that's my 2 cents.
Good luck and I hope you have great success.