Although the math is simple, there are a number of sail calculators available on the internet. One is http://www.sailingusa.info/parts_of_the_boat.htm
- putting in the I (44') and J (12.9') for a standard rig 345 gives a S/A of 284.
What does right-sizing the storm jib mean? Is your storm jib really 70% - I guess I don't know what % a storm jib "should be" but ours is by eyeball maybe half or less of the area of our 85% jib.
As to matching the main, assuming you have somewhat standard reef sizes, you'd need a third reef for use with a storm jib. Do you have an inner forestay for flying the storm sail?
You might want to seek out more commercial advice....