When using /drawtext -p with tabs it appears mIRC does not calculate the correct length for the text. Weirder, is that it varies from draw to draw:
alias testdt {
var %tab = $chr(9)
window -p @testdt -1 -1 1000 1000 consolas 11
clear -@ @testdt
drawtext -p @testdt $color(text) consolas 11 10 10 a %tab 106.87 %tab 383.14 %tab 254.82 %tab 475.6 %tab 197.66 %tab 613.85
drawtext -p @testdt $color(text) consolas 11 10 25 abc %tab 106.87 %tab 383.14 %tab 254.82 %tab 475.6 %tab 197.66 %tab 613.85
drawtext -p @testdt $color(text) consolas 11 10 40 abcdefg %tab 106.87 %tab 383.14 %tab 254.82 %tab 475.6 %tab 197.66 %tab 613.85
}
results in something akin to: