There's a guide on converting ag to extract here:
http://www.aussiehomebrewer.com/forum/inde...;showarticle=97
What you are doing is something in between.
All figures in the following are arbritrary - they are for illustrative purposes only. Where you see gravity figures, I have made them up - follow the principles, not the numbers.
Say for example you have an AG brew that uses 5 kg of pilsner and 1 kg of munich. (They are your base malts - if there are spec malts in the recipe then keep them the same)
The specified SG of this in the recipe is 1040, the OG of the boiled, cooled wort is 1055. The postboil volume is 20 litres. Your system loses 8 litres of wort over a 60 minute boil so you need 28 litres of preboil wort.
Let's says you can comfortably mash about half of this grain bill. That means you can make half the wort using half the ingredients in the same proportions. Therefore you can make 14 litres of wort with your system (mashing and sparging).
You therefore need to calculate how much malt extract to water will give you the other 14 litres of 1055 wort. Use a spreadsheet or software to do this. What do you currently use to work out recipes and gravity? Beersmith? Promash? Ianh's sheet? Beerrecipator? Beertoolz? Nothing?
The malt extract will replace only the base malt - the spec malts and hops additions should be kept exactly proportionate to the recipe. You do need to take into account efficiency and adjust (the recipe might suggest 90% but it's unlikely a first time mash brewer will hit that). I don't want to get too technical but to start I would aim for a figure of between 60 and 70% and scale the recipe accordingly. If your brew ends up too strong you'll be able to dilute.
Hoping not to insult your intelligence but you do know the difference between spec malts and base malts and that the grain needs to be cracked first? Very important.
Very simply all you are doing is dropping out some of the pale base malt in the original recipe and replacing with pale malt extract.
