Let me first say that in no way am I having a go at anyone but I am hopefully trying to give you an insight into how to measure as accurately as you need to when making a guitar. I seem to see a lot of people having problems with measuring where the bridges are to be positioned and while the advice is helpful it seems to be a little over the top and people are trying to polish a turd, excuse the French, and some of the figures have me falling off my chair laughing. My background is in Toolmaking so measuring accurately is pretty important. The above picture of the rule over the frets has a parallax error as your have to be directly over the middle of the fret and that picture isn't as it seems to be over roughly the 11th fret due to where the focus is best. Other little bit of advice is when you use the 1mm scale you can only measure plus or minus 1mm, any size smaller than that is not a measurement but a guess. If you want to measure finer use the 0.5mm side but again you can only measure to plus or minus 0.5mm. Guessing a size is not measuring, if you want a finer measurement you will need another device, just use a steel rule as it is more than accurate enough. We all know that these kits are not the most accurate and we are hoping the nut is parallel to the frets but are the frets parallel to each other? No one checks but we are trying to tie down a bridge position to the second decimal point, Why? You cannot measure it with a steel rule any more accurate than plus or minus 0.5mm in reality. That is why the bridges have adjustment and the adjustment is more than enough to cover what you need as long as you are pretty close, plus or minus 0.5mm is heaps. Then you have the position of the 12th fret which is where you intonate but if the position is, like the rest of the kit, not accurate then the scale length is then tied to that measurement and not the kit size. Here is how I measure my position as told to me by a luthier and after I was told I used common sense to try and prove/disprove why the procedure was correct. I use a slotted straight edge and adjust the neck flat, hopefully there are no high frets between the nut and the 12th fret, if there is then take off the high spots so the rule is touching the top of the 12th fret or in a pinch just adjust the neck so the rule touches the 12 fret. I adjust my bridge saddles in the middle 2 slots to their maximum and minimum positions. To measure where the 12th fret I put the guitar on its side and place the rule against the nut, shine a light from underneath and see where the rule touches the fret and read off the measurement from the rule. Double that and that is the scale length. I then place the rule against the nut and with the size obtained from the 12th fret measurement I then place the bridge so that size is, as near as possible, in the middle of the 2 saddles. You can use a 150mm rule along with the metre rule, place some tape on the measurement you got from doubling the 12 fret measurement and use the smaller ruler to measure each side to the 2 saddles. I use some masking tape on the top surface of the guitar and mark the front edge of the bridge and then you can mark where the holes are to drilled to mount the bridge. That is how I do it and after 14 guitars I have never had a problem. All my guitars intonate correctly which is all you can ask from this quality of kit. Don't get flustered about trying to obtain any measurement to a second decimal point as more than likely you wont be able to measure it accurately and it is not necessary.