On-chip performance counters are gaining popularity as an analysis and validation tool. Most contemporary processors have between two and six physical counters that can monitor an equal number of unique events simultaneously at fixed sampling periods. Through multiplexing and estimation, an even greater number of unique events can be monitored in a single program execution. When a program is sampled in multiplexed mode using round-robin scheduling of a specified event set, the number of events that are physically counted during each sampling period is limited by the number of counters that can be simultaneously accessed. During this period, the remaining events of the multiplexed event-set are not monitored, but their counts are estimated. Our work quantifies the estimation error of the event-counts in the multiplexed mode, which indicates that as many as 42% of sampled intervals are estimated with error greater than 10%. We propose new estimation algorithms that result in an accuracy improvement of up to 40%.