Trigger prototype activating too soon

[SnowDuck]

Hello all! I am using Zabbix 3.2.6 on Ubuntu 16.04. I am monitoring the used disk space in percentages. I have created a calculated check for this using the formula:
"last("useddisk[{#HRSTORAGEDESCR}]") / (last("availabledisk[{#HRSTORAGEDESCR}]") + last("useddisk[{#HRSTORAGEDESCR}]")) * 100"

The calculated check works as intended and returns the correct value. I have made a trigger with the severity "warning" when the used disk percentage becomes bigger than 50%.
The trigger prototype looks like this:

Name: {ITEM.VALUE} Used! Less than 50% free space remaining on {#HRSTORAGEDESCR}

Problem expression: "{LLD templateercentageused[{#HRSTORAGEDESCR}].last()}>50"

Recovery expression:
"{LLD templateercentageused[{#HRSTORAGEDESCR}].last()}<=45"

I thought this would work, but the trigger prototype activated even though the used disk percentage is nowhere near 50%. When I hover my mouse on the warning, I get the following message:
"50.09 % Used! Less than 50% free space remaining on /mnt/datadisk". Which is not right, because when I look at the latest data it says 47,85% disk space used and on the monitored machine it says 48% disk space used. Is this a bug or did I do something wrong?

[SnowDuck]

This threat can be closed, I managed to solve this issue after alot of trial and error.