According the htop source code, my assumptions looks like they are valid:
(see static inline double LinuxProcessList_scanCPUTime(LinuxProcessList* this) function at LinuxProcessList.c)
// Guest time is already accounted in usertime
usertime = usertime - guest; # As you see here, it subtracts guest from user time
nicetime = nicetime - guestnice; # and guest_nice from nice time
// Fields existing on kernels >= 2.6
// (and RHEL's patched kernel 2.4...)
unsigned long long int idlealltime = idletime + ioWait; # ioWait is added in the idleTime
unsigned long long int systemalltime = systemtime + irq + softIrq;
unsigned long long int virtalltime = guest + guestnice;
unsigned long long int totaltime = usertime + nicetime + systemalltime + idlealltime + steal + virtalltime;
And so, from fields listed in the first line of /proc/stat: (see section 1.8 at documentation)
user nice system idle iowait irq softirq steal guest guest_nice
cpu 74608 2520 24433 1117073 6176 4054 0 0 0 0
Algorithmically, we can calculate the CPU usage percentage like:
PrevIdle = previdle + previowait
Idle = idle + iowait
PrevNonIdle = prevuser + prevnice + prevsystem + previrq + prevsoftirq + prevsteal
NonIdle = user + nice + system + irq + softirq + steal
PrevTotal = PrevIdle + PrevNonIdle
Total = Idle + NonIdle
# differentiate: actual value minus the previous one
totald = Total - PrevTotal
idled = Idle - PrevIdle
CPU_Percentage = (totald - idled)/totald