I think there is a bit of misunderstanding as to image resolution. The number of pixels is the important factor in determining the maximum high quality print size. Pixels is the absolute measure of information in the picture. You can print at whatever DPI you want, but the higher the DPI the better the image quality and the smaller the picture will be. Percents are only relevant to adjustments being made to DPI, canvas/image size or pixel count.
Increasing the number of pixels artificially to have a higher size and "higher DPI" will not increase the quality at all. It will actually decrease it unless it is an even multiple of the original number of pixels since the software will have to guess at the in between pixels through a lossy process called interpolation. You don't gain anything in terms of quality unless the printer is incapable of making a smooth interpolation.
To achieve a high quality print, the general requirement is 300 dpi which means that to get a 48 by 18 inch print, you would need about a 78 MP image. For a 16 by 20 image you need about 24 MP. The image you have is only 4.5 megapixels and is only going to be sufficient to get a high quality 11 by 4.