Since I am noob in map making I just made a .jpg that you need to put in your import folder and load the map with the "disc" symbol in your W:A editor.
Place your worm like mine shown in the replay, each crosshair movement is counted as 1. This showcases 4sec fullpower notching. Like Mab already said, the weapon moves at 5,9,13 etc... this is important to memorize so you don't need to count the little tiny crosshair movements but rather the big steps (you memorize the angle for each notch with practice anyway)
Ok now for actual combat use you need to move your screen just so you can't see your worm anymore that's how you measure your distance to an opponent worm if he stands on 9 for example you can just notch to 9 (by single steps) or by going straight to the 2nd weapon movement which is 9 too.
The pixel values are roughly:
Step1: 116
Step2: 229
Step3: 351
Step4: 464
Step5: 594
Step6: 702
Step7: 814
Step8: 927
Step9: 1024 (see why 1024xxxx is such a good resolution?
Step10: 1138
Step11: 1236
Step12: 1340
Step13: 1434
Step14: 1524
Step15: 1610
Since I work on 1680x I stopped there but it covers almost the entire map, you can switch to 1920 and make it cover the whole map if you want.
Also note: the further away your worm stands the least effective a 4sec FP gets, you can clearly see in the replay that the explode radius shifts higher and higher, that means if your opponent stands higher or lower than you, you have to keep that in mind, this will also be covered in the next part where I will explain 3sec nades.
P.S.: HOW THE f@#! DO I RECORD W:A? fraps doesn't work I would just comment the stuff I am doing on the fly rather then write down essays.
Keep in mind these values are not 100% accurate and the pixel values may differ approx 10 pixels around but you will definitely get the idea by watching and reading this.
I will do 3sec tomorrow or later this evening.