I think the mistake you're making is entering time for both ownship and contact when you're wanting the TSD to return time to intercept. Entering time at both nodes (or ownship and contact) will tell Mobo you have two observation times and you're trying to solve for speed, which isn't what you want to do.
For a simple intercept course solution, you don't need to adjust map scale or enter time to get an accurate intercept course.
Data Required:
1) Plot ownship position/heading/speed
2) Plot target position/heading/speed
Process:
After plotting ownship and target positions and motion vectors, hover cursor over target and press I to show intercept location (assuming intercept is possible). If nothing shows, then there is no intercept.
After you have the intercept plotted you can plot a node on the intercept location and connect it to ownship (hover over node and press spacebar) and then press B to display bearing. The bearing will give you both true and relative headings to intercept.
Intercept Tracking Node:
The InterTrack tool was added in an update that allows you to assign a node to track with the calculated intercept for any target. To assign a node to track with the target intercept point, right click a node and select InterTrack from the quickmenu, then select the target (not the intercept, but the target) you want the node to track. Now as you move ownship and target you'll see the node tracks with the intercept for the unit. The node will track the intercept point regardless of whether or not the target unit has the intercept displayed.
TSD Calcs:
If you want to do TSD calcs (time-speed-distance) to determine the time or actual distance until the intercept point you will also need the scale settings adjusted accordingly, which it sounds like you've already done. The TSD calcs require any 2 of the 3 elements in order to display the third. Now as a basic example, just leave the time as zero for ownship and add a node anywhere on the map and connect it to ownship. Hover the cursor over the node and press Y. This will show you the time it will take to cover the distance given ownship's speed. If you enter a time for ownship, it will adjust the time accordingly so if it takes 22 minutes to intercept and you entered 120 as the time, it will show you 1:44 as the time of intercept. If you left ownship time as zero, it would just display 22 minutes.
The other usage for TSD calcs is to plot two observation nodes and connect them to each other. This represents a distance traveled by a contact. You then enter a time for each of the nodes (hover over node and press T). Now you have distance traveled and time, so the missing component in the TSD calc is speed in this case. When you hover over one of the nodes and press the Y key it will tell you the speed.