One of the difficulty in getting wheel sizes that fit within the wheel well with out rubbing on the supension on the inside or sticking out to far on the outside is that the Tiger front end is relatively wide-not much room to juggle backspace or offset.
The stock 4 1/2 wide 13" rim has a 25.4mm offset and 3.75" backspace. So, the way I see it, if a 5 1/2" wheel had the same offset, it would protrude toward the outside of the wheel well somewhat. Maybe a 1/2 inch? So, to get the 5 1/2" wheel to the same position in the wheel well, you would need less offset. So, 20mm might get you there for the same look. But, this would push the wheel into the inside of the wheel well and potentially rub on the suspension.
The more the positive offset (less backspace) the more the wheel sticks out towards the fender and increasing the width of the wheel increases this effect. You could look at the installed Lat 50 5 1/2 inch installed rim and tire and see if there was room for greater backspace or less offset. There are some tools that allow you to see if certain size wheels fit in the space available. Sometimes this really limits you to what, if any, rims are available.
Dale's front suspension has about the same width as the stock Tiger, so finding wheels that fit over the brake calipers and close to the suspension but are still appropriately inside the fender is difficult depending on what brakes and size wheels are choosen. I made a mistake and ordered 13" disk brakes on my front end. You can't fit 15" rims over the 13" calipers, so you have to go with 16" or 17" rims. Rims are sold by width and size with a small selection of offset for each style. With a requirement of 5" backspace to keep the wheel inside the fender, there was a very limited choice in wheels. There was more choice in 17", but 16" was already pushing it for a Tiger. There is also much less tire choice for 16" vs 17." So if I were doing it over, I'd go with 12" brakes and 15" rims. It's probable that the designers of the 15" Lat wheels made the best compromise they could. Cliff