I am experimenting with an online seal generator, the text I have entered is 一千五百 and I have noticed that the characters go first from top right to bottom right, then from bottom left to top left. Is this correct?
The general principle is from top to bottom, from right to left, but the format can be very flexible. For a four-character seal, there are 6 ways to arrange the characters (see picture). Seal calligraphers normally pick the arrangement that is most aesthetically pleasing, depending on how the character is written & number of strokes per character, etc. The only exception I'm aware of is for 4-character seal, the 2nd format in the picture breaks the right-left rule but that particular one has a particular name called 回文布排 ie "anti-clockwise" that is for 4-character seal only.
picture taken from http://www.shufazidian.com/shuji/article.php?acid=21&arid=41
This is correct. It is called 竖排.