The rocker arm ratio multiplies the cam lobe list at the valves. Example, a stock HP cam has 0.450" advertised valve lift using the stock 1.5:1 ratio rocker arm. This means the cams max lobe lift is 0.450/1.50=0.300".
If a 1.7:1 rocker arm is used, the lift at the valve is now 0.510". Because the rocker arm multiplies the lobe lift, any time the tappet is off the cams base circle the valves will see the extra lift, so the engine thinks it has a slightly larger cam.
There are always trade offs. The extra lift will mean the heads, springs, and piston to valve clearances need to be checked. Hopefully I'll have the rockers later next week, and I'll start my experiment. I have some pushrods that might work, but I won't know till I get the heads on the engine.

Also, stock rocker arms are not adjustable, and usually the true rocker ratio is less than 1.5:1.