Hello,
We have tested one of our domain which uses SSL :
To use your SSL with Cloudflare, you will need to change the DNS to cloudflare (provision in full), and choose SSL: 'Full' or 'full'
Support :
https://support.cloudflare.com/hc/en-us/articles/200170416-What-do-the-SSL-options-mean-
https://support.cloudflare.com/hc/en-us/articles/200170566-My-SSL-isn-t-working-Why-not-