计算适合矩形的最大字体大小?

我试图找到一个给定字符串的最大字体大小。算法的目标是用尽可能大的字体填充尽可能多的矩形。我的方法——从我在网上找到的方法修改而来——做得不错,但它通常不会填满整个rect。我很想看到一些关于如何改进这个算法的合作,以便每个人都能从中受益:

-(float) maxFontSizeThatFitsForString:(NSString*)_string 
                               inRect:(CGRect)rect 
                             withFont:(NSString *)fontName 
                             onDevice:(int)device 
{   

    // this is the maximum size font that will fit on the device
    float _fontSize = maxFontSize;
    float widthTweak;

    // how much to change the font each iteration. smaller
    // numbers will come closer to an exact match at the 
    // expense of increasing the number of iterations.
    float fontDelta = 2.0;

    // sometimes sizeWithFont will break up a word 
    // if the tweak is not applied. also note that 
    // this should probably take into account the
    // font being used -- some fonts work better
    // than others using sizeWithFont.
    if(device == IPAD)
        widthTweak = 0.2;
    else
        widthTweak = 0.2;

    CGSize tallerSize = 
           CGSizeMake(rect.size.width-(rect.size.width*widthTweak), 100000);
    CGSize stringSize = 
           [_string sizeWithFont:[UIFont fontWithName:fontName size:_fontSize]
               constrainedToSize:tallerSize];

    while (stringSize.height >= rect.size.height)
    {       
        _fontSize -= fontDelta;
        stringSize = [_string sizeWithFont:[UIFont fontWithName:fontName 
                                          size:_fontSize] 
                             constrainedToSize:tallerSize];
    }

    return _fontSize;
}