50% sounds reasonable to me, but obviously the higher the better if your base cost allows you to stay within a reasonable price range for your clients. My current plans are calculated for 30%, but I still realize certain costs I forgot every day so I might nodge it up to 50%.
Also, I want to be prepared for the future. Being a reseller (like me) is generally a lot cheaper than renting or owning your own server. The last thing you want to do is having to increase your prices by 200% because the basecosts of running a dedicated server are 200% higher than your reseller package. I'd rather lower my prices should the dedicated server end up cheaper than expected. No nasty surprises and potentially leaving clients, that way.