if you know how to make a .conf file your 90% done already

in the .conf file just write "reservebalance= 50" it might also be "reservebalance=50" (not sure if space is needed or not)

also if all your tx's are bigger than what you reserve then it will reserve the whole tx not just the requested amount  (i think thats how they work) so if you reserve 50 coins but smallest tx has 100 coins in it then 100 coins will stay reserved