How do I calculate brightness of LED strips equivalent to incandescent light bulbs?
You need to know the light output. LED strips (and other LED products) usually give this in lumens

Typical values for incandescent light bulbs are

Watts    Lumens
40       450
60       800
75       1000


Since LED products vary considerably in efficiency, I can't give figures for them.

To do this properly you need to understand the difference between luminous flux, luminous intensity, illuminance and other measurements.