Very small notch (and feed width) for a inset-fed microstrip antenna, is it correct?

Hello!

I'm designing a microstrip antenna which is going to be fabricated by drilling it out of a FR4 laminate with the specs:

= 4.3
h = 0.08cm

For the frequency 866 MHz (using it for RFID), the parameters i"ve calculated are:

W = 10.640 cm

= 4.230
L = 8.346 cm
inset length, y0 = 2.875

So far i think it all looks good, but when i"m calculating the inset width and feed line width i'm getting some odd numbers.
The calculator over at emtalk.com gives me 0.156 cm microstrip line width while the formulas in Balanis book (3rd edition page 825, chapter 14.2.1) gives me 0.123 cm width.

These seem awfully small, is that because of the substrate being so thin or have i made an error calculating it?

Edit: Maybe this was the wrong forum? Please move it or direct me to the right one if so! :)