You will need to use a long arm ruler, not a rotary cutting ruler. These are at least 1/4" thick so in addition to it being round, the part that attaches the round bottom needs to be high enough so that your ruler will fit. Probably not making myself too clear...
Couldn't find an exact photo, but here's an open toed foot that will illustrate:
http://www.sewingmachinesplus.com/tin_foot.php
See how high that shaft is? It needs to be really high to clear the ruler.
The ruler has to be thick so when your foot hops, the ruler can't slip under the foot.