Adjusting for gauge

Hello,

I hope someone can offer me some advice!

I was hoping to begin a Frankie Sweater, but in Donegal tweed. The gauge I’m getting is 2 stitches lower than the prescribed. I’m getting 18st for 10cm when I need 20.

So my gauge is like 10% too big.

Does anyone know if I’m ok to adjust the number of cast on stitches accordingly, and work per the general instructions from there (increases and decreases), or will the proportions be too far off?

Any guidance much appreciated.

Thanks!