The way you use this calculator is you find the casting speed of your spell and then look at the 3 dex amounts to the right of it. Each of the amounts is a breakpoint for you to hit. If you can't hit the breakpoint, then any of the points used are wasted, and you will not gain any advantage for using the extra dex and should use the points elsewhere.
For example if your spell cast speed is 2.4 then you want ti try and obtain 392 dex and if you cant lower it to either 350 or 308.
Dex Break Points by Casting Speed
10% Casting speed Credit Slajzer
2.0s ---> 392 dex, 344 dex, 337 dex
2.1s ---> 356 dex, 354 dex, 308 dex
2.2s ---> 374 dex, 368 dex, 320 dex
2.3s ---> 386 dex, 380 dex, 332 dex
2.4s ---> 392 dex, 350 dex, 308 dex
2.5s ---> 362 dex, 319 dex
2.6s ---> 374 dex, 332 dex
2.7s ---> 386 dex, 344 dex
2.8s ---> 392 dex, 356 dex, 320 dex
2.9s ---> 368 dex, 362 dex, 332 dex
3.0s ---> 374 dex, 338 dex
3.5s ---> 356 dex, 326 dex
9% Credit to Gydd
2.0s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
2.1s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
2.2s ---> 326 dex, Did not test higher
2.3s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
2.4s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
2.5s ---> 325 dex, 367 dex
2.6s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
2.7s ---> 350 dex
2.8s ---> 356 dex, 392 dex
2.9s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
3.0s ---> 346 dex, 380 dex
3.5s ---> Did not complete (estimated 6 dex higher than the 10% breakpoint)
Archery speed tests for 4.2 standard shot - new dex break
Results Tests below in reverse order. Number on the left is dexterity, number on the right is shot speed for 4.2 standard shot with 10% archery speed (casting speed = same as archery). Note - there is some variance because the parser only counts to the nearest second, plus server lag. This means that tests for the same dexterity will have tiny variance.
392 - 1.75
392 - 1.77
392 - 1.76
393 - 1.76
393 - 1.77
394 - 1.76
394 - 1.76
397 - 1.76
BREAK
398 - 1.72
404 - 1.71
414 - 1.69 (odd variance)
415 - 1.72
473 - 1.71
473 - 1.72
So the new dex objective for archers should be 398.
It'll give you a roughly 3.1% archery speed increase over 392 for standard shots in version 1.121.
Use this link to view the original post for the Dex Calculator: https://docs.google.com/spreadsheets/d/1sPjFvpR40HMS8_1F7leojuz0hh1qCYVaRe_pkWjOyqw/edit?usp=sharing