You can use math to calculate an exposure correction because the amount of each step is a known amout. Each step to increase or decrease exposure has a value of 1.414. If you need to adjust more than one step, you have to multiply the steps. It's logarithmic. 

1 step is 1.414 exposure
2 steps 1.414 * 1.414 = 1.9999 so you should double exposure. 
3 steps 1.414 * 1.414 * 1.414 = 2.827 means about 3x exposure. 
4 steps 1.414 * 1.414 * 1.414 * 1.414 = 3.99758 which means 4X. 

See, it makes sense.

Use a 21 Step on every screen for visual feedback

You will get visual feedback of the invisible, UV energy that cross links your stencil when you put a transmssion gray scale on every stencil. You will notice when it washes out differently and you will know your lamp or stencil has changed. You will know when to change your exposure to get back to a minimum Step 7.

For visual feedback like a Step Speedometer, put a 21 Step on every screen you expose for the rest of your life.