addition with an unknown range

i am relatively new to excel...so the answer is probably under my nose but i don't know...

setting up spreadsheets to track dollar amounts. column c is the amount, and column d is the amount in column c divided by two. not a problem there. here is the problem. is there a way to setup the final formulas, (=sum(c5:c100) and (=c100/2) where i don't have to plug in the final line? each month varies in length of entries, and would like to have a running total as the month goes on, instead of making sure that all have been entered and doing the total line manually.