X Server / Tightvnc tunnells

OK this is a port tunnelling thing I'm not seeing how to fix... thanks in 
advance for taking a gander.

I have two machines behind seperate firewalls, and a third server that both 
can access.

"grue" and "smeg" are behind different firewalls, and i need to see grue from 
smeg using a gui type thing. I was trying to do tightvnc and now i'm looking 
at ssh -Y .

In order to ssh to grue I had to do sit at the computer physically and forward 
port 22 on it to some arbitrary port on "kron" (my server). Then from smeg I 
ssh to kronos, then i ssh with -p ??? where the ??? is my arbitrary port. 
That works lovely.

But now I can't seem to figure out how to forward the ports so i can do 
tightvnc... Perhaps someone can give me some step by step on what I need to 
do to get the correct  ports forwarded and then how to access those ports so 
I can get to grue from smeg through kron.

thanks guys I really appreciate it.