Drawing a ruler in iOS - accurate 1 cm on screen - how to draw lines 1 mm apart?

I am trying to draw a ruler, for any available iOS device, with accurate 1mm distances between start of lines/ticks. Usually i would get the PPI and calculate my distance of the pixels. Using Objective-C, it does not seem to work this way.

linesDist should contain my 1mm distance in "screen coordinate pixels".

Any ideas, how i can achieve this?

my basic code looks like this: RulerView.m, which is a UIView:

-(void)drawRect:(CGRect)rect
{
    [[UIColor blackColor] setFill];

    float linesDist = 3.0; // 1mm * ppi ??

    float linesWidthShort = 15.0;
    float linesWidthLong = 20.0;

    for (NSInteger i = 0, count = 0; i <= self.bounds.size.height; i = i + linesDist, count++)
    {
        bool isLong = (int)i % 5 == 0;

        float linesWidth = isLong ? linesWidthLong : linesWidthShort;
        UIRectFill( (CGRect){0, i, linesWidth, 1} );
    }
}

EDIT ppi detect (really ugly), based on the answer below:

float ppi = 0;
switch ((int)[UIScreen mainScreen].bounds.size.height) {
    case 568: // iPhone 5*
    case 667: // iPhone 6
        ppi = 163.0;
        break;

    case 736: // iPhone 6+
        ppi = 154.0;
        break;

    default:
        return;
        break;
}

iPhones (with the possible exception of the iPhone6+) are 163 "logical" points per inch. Obviously phones from 4 onwards have double or more the resolution but that doesn't make any difference for the coordinate system.

1mm therefore is 163/25.4 or approximately 6.4. iPad is 5.2 points per mm, iPad mini is the same as iPhone.

-(void)drawRect:(CGRect)rect
{
    [[UIColor blackColor] setFill];
    float i;

    float linesDist = 163.0/25.4; // ppi/mm per inch (regular size iPad would be 132.0)

    float linesWidthShort = 15.0;
    float linesWidthLong = 20.0;

    for (i = 0, count = 0; i <= self.bounds.size.height; i = i + linesDist, count++)
    {
        bool isLong = (int)count % 5 == 0;

        float linesWidth = isLong ? linesWidthLong : linesWidthShort;
        UIRectFill( (CGRect){0, i, linesWidth, 1} );
    }
} 

You want to use a float for i in order to avoid rounding errors when adding the distances up and to avoid unnecessary conversions.