Hello everyone.
To draw seamless orthogonal tiles I just set the step in preferences to a fixed values (e.g. 200px if the tiles are 200x200), and when I draw an object at the edge of a tile, I just duplicate it and move the copy of 200px right/down, so there is a repeated pattern for every tile.

Let's say that I have an isometric tile with a grid of 30%, and every edge of the tile is 147px. This means that to make a seamless tile I have to move every duplicate of 85px down and 147.224px to the right. But in the preferences, it's not possible to select two distinct values for the horizontal and vertical steps. Ok, I could just use an orthogonal grid and then rotate it, but it's easier for me to start with an assonometric grid. Is there a way to achieve what I am trying to do?

Any chance this helps?
http://tavmjong.free.fr/INKSCAPE/MANUAL ... tries.html

(The hexagonal tiles in particular.)

I'm a little confused with what you're doing, because you start out talking about orthogonal, but later you mention it's isometric. But just in case this might come in handy.

Edit menu > Clones > Tiled Clones might be an option, if I understand correctly what you're asking. Rather than repeating duplicate/move many times, Tiled Clones creates all the clones at once, and automatically places them in whatever kind or array you want.

Unfortunately, I along with many others find it difficult to figure out how to set the Shift tab. There have been many requests for other units besides percent. But you do seem to have enough math skills to sort it out. So fwiw.