Tell them you will take market rate and see what they offer. Thats always the easiest way to do it. If you give them a figure they will always try and knock you down. It doesnt matter if you are on way less than your market value, they should give you what you are worth. Find other job specs out there and use that as justification.
I assume if you are thinking of adding upto 45% to your current figure you are on less than market rate? To put it into perspective, when I moved from permie to contracting, I tripled my take home. Now this is extreme, but when I then went back to permie again only 1.5yrs later, I am now on more than double what I was when last permie. Its all down to your market value. My original employer didnt agree my market worth so I left and now work for someone willing to pay my market rate.