Timberland PRO Men's 6-Pack Half Cushioned Crew Socks, Black, Large

Q: Cover calf for slip on boots?

These socks do not fully cover the calf. They go a little past the bottom of calf (depending on your height). Purchased for my brother for his work boots and he loves them.

Q: How tall are the socks (heel to opening)?

10 inches from middle of heel to opening

Q: percentage of merino wool?

I don't have specific numbers in response to this query. But I do know that these are some of the most comfortable socks I've ever had the pleasure of wearing. They don't feel thick, but they feel warm and "breathable" while providing a comfortable fit.