Choosing new differentials, what power to pick?

Hey my old glasses are broken so I am going to the optometrist for new differentials. My current prescription is -4.00 for both eyes with no astigmatism, so what should I get for my differentials?

To figure out the measurement for your close-up glasses note that they have a lower correction than your reduced lenses, the difference between them being usually between 1 and 2 diopters.

Some rough example calculation with a value that is inadequate for close-up glasses:
We know that 100/cm = diopters and 100/diopters = cm. Here cm is your distance to blur. -4 glasses is your full correction, i.e. 20/20 vision. You will be myopic by 0.75 if you wear -3.25. (4-3.25=0.75).
Now 100/0.75=133.33 cm.
I don’t think your screen is closer than 133 cm.

Check out the wiki article on differentials for more info: Close-up glasses - Reduced Lens Wiki

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