Question regarding penelope 10/20 Canvas and DMC Perle Cotton Thread

Hello,

I just received a medival tapestry in blues and reds which I think
would look great in Perle Cotton thread. I plan on doing the
needlepoint in petite point by splitting the 10/20 penelope canvas. I
have just finished another tapestry using this same technique but
using DMC floss in which I used 4 strands of floss which worked well
before but did run a little thin near the end of the 18" long pieces I
was using. But I don't know whether I need to buy #5 or #8 Perle
Cotton. Does anyone know which size will be closest to the 4 strands
of regular floss that I used previously? I really need some help and
advice on this, because I have to order all the floss via the web at
once, and it will be non-returnable. Unfortunately, I can't see the
thread before I order it since it's not available where I am located.
Also, can anyone tell me whether they think the Perle cotton will look
good in a tapestry or will it be too shiny?

Thanks in advance for any advice and opinions!
c_bowlin