You can run the traffic shaper wizard and select Priq as the algorithm, punch in your upload and download speeds (actual; not rated) accordingly.
Run through everything (no need to check anything).
You should have a simple priq parent queue for WAN and for LAN.
Go to Firewall -> Traffic Shaper.
Click 'LAN'.
Click 'Add New Queue'
Set priority to say 7.
Name it as qAck.
Add queue again.
Set Priority as 5. Set as Default Queue.
Name it qDefault.
Add another queue.
Set Priority as 1.
Name it qLow.
Repeat for WAN tab.
Now go to Firewall -> Rules.
Click Lan tab.
Click the 'e' button beside the 'Default allow LAN to any rule'.
Scroll down till you find 'Ackqueue/Queue'.
Set to: qAck/ qDefault
Click Save.
Now click the '+' sign beside the rule.
Go to 'Source'. Change from 'LAN subnet' to 'Single Host or Alias'. In the box below, fill in the IP address of the computer to throttle.
Scroll down to 'Ackqueue/Queue'.
Set to: none/qLow.
Rename the Description to 'Throttle Download'.
Click save.
In the LAN tab, you will now see both rules. Check the box to the right of 'Throttle Download' then click the Arrow button beside 'Default allow LAN' rule to move the throttle rule above it.
Click Save. This settles the upload throttling.
Now for download throttling. This gets slightly trickier.
Click on 'Floating Rules' Tab.
Click Add new rule (+ button).
Check 'Apply the action immediately on match' box.
Under interface, choose WAN only.
Set direction to 'In'.
Set Protocol to Any.
Set Source to Any.
Set Destination to Single host with IP of the download machine.
Go down and set the queues to none/ qLow.
Set Description to 'Download throttle'.
Save the rule.
Under floating rules, duplicate this rule.
Change Destination to 'Lan subnet'.
Go down and set the queues to qAck/ qDefault.
Set Description to 'Default CatchAll'.
Save the rule.
No re-ordering is necessary. Just click the save at the top of the page.
That should do the trick.