How does the string tension on a tennis racquet work?

I'm getting a new racquet and I plan on getting a combo of synthetic and natural gut, the natural being the main string and the synthetic being the cross strings. I have several questions, I wanted to know what specific numbers mean, I was planning on getting the mains strung at 57 lbs and the cross at 55 lbs but I dont know if that is too low of a number. I'm an intermediate player who uses a lot of top spin and has a lot of power. Also, is this a good combination of strings and should they be switched around with the syn as the mains and the gut as the cross? Thanks! 5 stars for all the help i can get!