DP to pixel convert in iOS(ipad) issue.

I wanted to find pixels cover by AdBanner on screen which is of 90 DP and shift viewport of camera to create space for bannerAd. After reading blogs and all i found
pixels = dp * ( device_dpi / 160 ) I used this calculated pixels to change viewport of Camera. I changed camera.rect's Y by normalized pixel-answer of above.

in my case
dp = 90
device_dpi = 246 (ipad)

but calculated pixels are less than actual bannerAd height (of 90 DP). Any idea which other factors may affect this?