DPI/PPI is the 3rd part of a 3 part equation, and is in fact the least important.

Agiel is exactly right - set manual dimensions at 2480x3508 (going by what Photoshop calls A4 size - I'm not arguing with you Agiel ;-)) and it doesn't matter if it renders at 10dpi - the image's dimensions are 2480x3508 pixels.

You can set/change the dpi/ppi to 300 in Photoshop (or PaintShop Pro - or whatever you use for corrections and printing).

Wabe is also right - you can go down to 225 and never see a difference - in which case you set your target image size to 1860x2631. When the publisher tells you they want an 8x10@300dpi (2400x3000 image) it's just for quality safety. I've also heard the number 212 passed around - printers print at 150 lines per inch unless they are a specialty printing house.

Having worked in many photo labs, if you are taking it to a minilab that prints from digital (using a Fuji Frontier, or Noritsu minilab, for example), you can safely print an image in A4 size that is based on 150dpi - set your image target to 1240x1754 - no kidding! And for most inkjet printers you won't see the difference either unless you look REALLY close. So close that you pause, and suddenly realize NO ONE is going to do what you're doing right now...

Good luck - hope that helps-
-Lew ;-)