Intensity divided by 254 gives me a percentage.

No, it doesn't. it gives you a ratio. But, the division is integer division.

Intensity calculates on the fly to be somewhere between 50 and 30.

Dividing 30 by 254 gives 0. Dividing 50 by 254 gives 0.

If I take my data into excel:

Sure. Because excel doesn't distinguish between integers and real numbers. C and the Arduino do.

In excel, (50/254)*203 = 40. In C, (50/254)*203 = 0, because 50/254 = 0, because both values are integers.

Now, (50.0/254.0) * 203.0 will give 39.96063 as a result, because all the operands are floats.

By the way, you would get different results if you changed the order of the operations. Multiply first, then divide. The result is still integer math, but the result would not always be 0.