T condition equates to 850-1000 MPa Ultimate Tensile Strength. Based on one of the conversion tables I use, that's 25 – 32 Rockwell C.

Any of these strength/hardness conversions for steel are approximate, reality may vary from grade to grade.

Not found a simple list on the internet, but Tubal Cain's Model Engineer's Handbook, is useful for this and many other home workshop questions. Well worth £7.10 (cheaper second-hand)

Machinability is a complex subject. A metal's hardness is just one of several factors and not one I'd draw conclusions from. Grain structure is a major influence and rarely mentioned in specifications, perhaps because heat treatment alters it. Instead specifications usually describe a metal's machinability from stock using words like 'poor' or 'average'. Silver Steel is 'good' while EN24T is 'excellent'. EN3A is 'average'.

Bottom line, easy to get good results from 'excellent' metals like EN1A-Pb, expect bother with 'average' such as EN3A, and grief from anything described as 'poor'. In all cases, finish depends on correct tool, depth of cut and cutting speed.

For most small workshop jobs requiring hardening, Silver Steel is safer than EN24T. Machinability is nearly as good as EN24T, but Silver Steel heat treats with less fuss. In most cases either would do, and I'd go with the one that makes the work easiest! Silver Steel.

For comparing the properties of various steels to get a feel for what they're about, I find Machinery's Handbook convenient. The US steel classification system follows the effect of increasing Carbon more obviously than other standards, and Machinery's Tables also relate each steel to it's intended purpose. The European and other steel classifications follow different logic, no problem if you understand it, but not beginner friendly or easy to decode from scratch. The problem with selecting US Steels outside the US, is translating their numbers into a local equivalent. Nothing is ever easy…

Dave