Recipe Tweaks

I was looking at some recipes for some PM i would like to do and i had a thought, I want to take a 5 gallon batch and turn it into a 7.5 or 8 gallon ect so is it as easy as just doing a simple % calculation example...

This is a 5 gallon batch to a 7.5
1 lb Brewers 2-Row Malt (Rahr)
6 lbs Wheat LME
1.5 oz Cluster Hops
1oz Coriander crushed
WLP840 American Lager Yeast
2oz Blueberry Extract
1 Whirfloc tablet (clarity)

So converted...

1.5 Brewers 2-Row Malt
9 lbs Wheat LME
2.25 oz Cluster Hops
1.5 Coriander crushed

and so on... now this one is simple with only a 50% increase but say i wanted to do a 9 gallon so an 80% increase or 6# x 1.8 = 10.8# and 1.5oz x 1.8 = 2.7 oz

Am I on the right idea....been up for about 23 hours so i could be off haha

Cheers

Cory

O and also do things like the 1 Whirfloc tablet need to be calculated the same way or can you still just use one? And still just one yeast starter?