Photoshop's built in algorithms are now much more advanced than the old 10% stairstep interpolation method. If your native resolution for printing is 180dpi or better, you may not need to upscale your images at all.
My camera raw settings are 300 dpi and with an uppscale to maximum file size there, how much could I push the files in PS?
How much resolution do I need for large prints?
Earlier I have done a 70cm x 100cm from one single D3 file, with maximum file size out of camera raw and then used crop tool in PS without typing in any resolution. That gave me an output of about 120 dpi.
To me, that image looked really good and tack sharp. I have no comparison so I am not shure how good these large prints can be.
Your camera raw ppi is a completely arbitrary and useless number. 180 ppi is considered a minimum by many people but as you discovered it is subjective and lower can be acceptable. I would upres to at least 180 ppi just to make sure stair-stepping is kept to a minimum in the print. There are to acceptable theory's to printing first if you have between 180-420 ppi in your intended print size the send it to the printer and be done with it. The other is to up or down res your picture to the native print resolution of your printer 360 ppi for epson and 300 ppi for canon, and 300 ppi is often assumed for print sent out. Personally I use the second method for anything I intend to be framed.
Photoshop's algorithms for up-res are as good up to about 4x the original size after that a third party application may do a little better.
