I tried that one but it the cursor went off screen.

Alternate solution before the prefs interface in 5.10 was created was what all advanced users did who wanted to manage the controls at host level. Visit the global_prefs_override.xml and edit the respective line, save and use the menu "Read local prefs file".

<max_cpus>4</max_cpus> number of concurrent science processes (A)
<max_ncpus_pct>100.000000</max_ncpus_pct> % of total processors/cores (B)
<cpu_usage_limit>100.000000</cpu_usage_limit> run % of time when BOINC is allowed to process.
<cpu_scheduling_period_minutes>120.000000</cpu_scheduling_period_minutes>

The A and B options work to always take lowest of the 2, thus even with 4 physical cores in the sample and 4 allowed, 99% would round down and allow 3 cores. Real cross platform user friendly and nobody can't deny ;>)