First I'm curious how you get a 70x90 inch print out of a lightjet. Unless I am mistaken, the largest lightjet page is 50x120.
Most labs run their lightjets at 200dpi with the 300dpi mode used only when very fine text is being printed. You will also find the hardware based upsizing/sharpening in the lightjet is going to be as good or better than doing the upsizing in photoshop.
I have tested the same file using 150, 200 and 300dpi and could not see a difference without using a loupe. It was minute enough that unless prints were side by side you wouldn't notice deficiency even with a strong loupe. Remember, your dealing with continuous tone not dots of ink. The equivalent sharpness is something like 10 times a 360dpi inkjet. It's to the point that if your file is prepared properly it just doesn't matter. Furthermore, with fujiflex paper you don't lose any detail to the texture of the paper.
Not many prints 90 inches wide are viewed from a few inches with a magnifying glass. I have a 36"x96" pano in the front of my gallery that I switch out 3 or 4 times a year. The current image is cropped from a single IQ180 exposure of a scene with an incredible amount of
detail that pretty much dares the viewer to nose up to inspect it. The corn field has enough detail that you can see the veins in the individual leaves and every blade of grass is discernible. You can even see the pot of flowers and curtain detail in the cabin's window. It is printed from a 150dpi file with the lightjet set at 200dpi.