In order to get a better idea and more precise layout when dry laying floor tiles, it is always advisable to use tile spacers at this early stage in your project. These are normally plastic or rubber crosses which can be bought by the bag at a fairly reasonable price, and are essential when finally laying your floor tiles in a bed of mortar. By using these as a temporary placement, you will certainly gain a more accurate projection of your tiled floor to be.
When dealing with ceramic floor tiles which are the most commonly used, it is best to work with either 3/16" or 1/4" inch tile spacers. These spacers give your final and exact width of your grout lines when inserted correctly, but they do come in useful during the tile projection process of dry laying. There are 1/8' inch tile spacers also available, but these are more prone to being used with either wall tiles or specialist granite and marble floor tiles, and I advise against using them if dealing with any other purpose than for these.
Any grout line spacing wider that 1/4" inch is not recommended, and if such a tile spacer is available which I have yet to personally come across, then I would highly advise you avoid at all costs. These will only result in grout lines that will look way to wide against your floor tiles, and really be of an impractical nature.
One final tip when working on your own and trying desperately to snap your chalk lines, involves the need to provide yourself with a dummy 'pair of hands'. If your subfloor is of plywood or concrete backer board construction, then by tapping in a nail or inserting a screw part way into your desired marker spot, you will have a temporary and precise starting point for which to project your chalk line.
If you are working on a solid concrete subfloor however where it is more difficult to insert a nail or screw without damaging the floor, or you simply don't wish to follow the above fixed point method, then as an alternative use a heavy object such as a stack of tiles. Just wrap the chalk line around the weight to hold it down firm and flat to the ground, and then repeat as above to snapping those lines with ease if you're flying solo.