I am going to throw this out there as I am not sure what you are working with. If this is your first all grain brew day, be sure to use a tried and true recipe which others report success with. There are a ton of great recipes in the Recipes tab of this site. Look for the ones which have the most comments. Those are almost always good recipes to work with. Besides knowing that the beer comes out well, these recipes include all of the key information you need like hop schedules, grain amounts, fermentation temps, etc. Formulating your own recipe is a challenging process if you don't already know your ingredients and what they bring to the table.
I personally have yet to make my own recipe from scratch and I have been brewing 2 years. Not that I have never adjusted my recipes to fit what I am looking for. We all do that, but starting from scratch is different. You can d it certainly, but you really may not know what you are going to get out of the batch. You could get lucky which would be great, but then maybe not. Given the cost of brewing I prefer to err towards safety at this point. Besides, there are so many great recipes allready out there I ahven't had time to figure that I need to create something brand new myself.
Just my $0.02. take it for what it is worth.